This application is related to commonly owned patent application Ser. No. 518,583 filed on July 29, 1983 by S. Panush for "Pearlescent Automotive Paint Composition", now abandoned, which application is a continuation-in-part of Ser. No 440,764 filed on Nov. 10, 1982 now abandoned. It is more closely related to commonly owned patent application Ser. No. 526,724, filed on Aug. 26, 1983 by S. Panush for "Multilayer Automotive Paint System" now U.S. Pat. No. 4,499,143. The foregoing patent applications describe the compositions and structures of improved paints which are particularly useful in the automotive industry.
U.S. Pat. No. 3,639,147 discloses an older basic multi-layer painting system which has found particularly wide use in the automotive industry and is commonly referred to as a basecoat/clearcoat system. According to this system, a substantially pigmented basecoat layer is applied to a metal substrate to provide aesthetically pleasing colors and to hide surface blemishes. For a particular pleasing effect, the basecoat will contain metallic pigments such as aluminum flake. A clearcoat layer is next applied over the basecoat. Typically this is a substantially unpigmented layer of polymer which "deepens" the color appearance of the basecoat and provides durability in that it seals the topmost pigment particles of the basecoat from the environment. In this system and in the others referred to herein, additional clearcoats are optional, for the same reason.
While the foregoing system has met with wide use and provides a substantial improvement over previous one coat systems (or systems where multiple layers of the same composition were applied to a surface) the need for further improvements has been evident. Particularly, there is a continuing desire to improve the durability of coatings, to provide new and exciting aesthetic effects, and in most recent years, to reduce the solvent content of paints for atmospheric environmental reasons. The above mentioned co-pending applications provide a substantial improvement in the desired direction. The new coatings of the patent applications are notable because they contain Richelyn pigments (trademark of Inmont Corporation); these are very fine mica flake particulates having extremely thin transparent coatings on their surfaces. Coated mica pigments are described in U.S. Pat. No. 3,087,829. The mica pigment is placed in the basecoat of the first two patent applications referred to above, to provide a pearlescent appearance in an automotive paint; and, the second topcoat is clear. In the system which is the subject of the Ser. No. 526,724 application, the basecoat contains a substantial amount of the Richelyn pigment, while the topcoat contains a relatively small amount. This composition of enamel provides a quite different appearance to a coating, compared to the Richelyn pigmented basecoat with a clear topcoat, and compared to an aluminum pigmented basecoat with a clear topcoat, i.e., the familiar commercial coating.
This application is concerned with the method of applying the above mentioned Richelyn pigmented coatings, and especially that of patent application Ser. No. 526,724. Of course, there are numerous well known methods for applying conventional coating systems to surfaces. Most favored in production situations are: two-fluid atomizing, wherein air provides the driving force to atomize and impel the paint toward the workpiece surface; simple pressure atomizing, wherein the paint is raised to a sufficiently high pressure for it to atomize upon issuing from an extremely small orifice; and, rotary atomizing, where the liquid is flowed across the surface of a disc or bell shaped rotating member and disintegrated by Rayleigh breakup at the edge. Electrostatic charges may be applied to the foregoing apparatuses to cause charged paint particles to move toward a conductive workpiece surface. In particular, electrostatically assisted disc and bell rotary atomizers have been particularly favored in the automotive industry. It is not unusual for air to be utilized to supplement the action of a rotary atomizer and electrostatic force, to more efficiently guide the atomized paint particles toward the workpiece surface.
Notwithstanding all the past experience and technology which is available for applying paint systems to automobiles and other articles, it has been discovered with the new coating systems that the desired aesthetic effects and durability are not fully realized under all atomizing conditions. As with prior coating systems, there is a criticality in matching appearance of parts coated at different times. Particularly, in automotive applications there is a need for matching between portions of a vehicle which might be coated by air atomizing and those coated by electrostatic rotary atomizing. High solids content mandated by environmental regulations adds to the problem by making spray parameters more critical. The variations in appearance, both within the use of a particular method and between different coating methods, have been found to be greater for Richelyn pigmented coatings than for aluminum pigmented coatings. In our work we have attributed this difference to the nature of the mica particulate. Thus, development work was undertaken to discover the phenomena underlying the observed variability and to determine how to satisfactorily reproduce coating appearance. And of course it is an important criterion that, whatever the appearance, the coating must satisfactorily perform its protective function as well.